This invention relates to photoflash lamps and, more particularly, to flashlamps of the type containing a primer bridge ignited by a high voltage pulse.
High voltage flashlamps may be divided historically into three categories: (1) those having a spark gap within the lamp such that electrical breakdown of a gaseous dielectric (e.g., the combustion-supporting oxygen atmosphere) is an integral part of the lamp ignition mechanism; (2) those having a conductive primer bridge that electrically completes the circuit between the lead-in wires; such primers are rendered conductive by additives such as acetylene black, lead dioxide, or other electrical conduction-promoting agents; and (3) lamps having an essentially nonconducting primer bridge that connects the inner ends of the lead-in wires and which becomes conductive, upon application of a high voltage pulse, by means of breakdown of the dielectric binder separating conductive particles therein.
The earliest high voltage flashlamps were of the spark gap type construction wherein an electrical spark would pass through the gaseous atmosphere within the lamp. The spark would jump between two electrodes, at least one of which was coated with a primer composition. Such lamps tend to exhibit poor sensitivity and reliability when flashed from low power sources such as the miniaturized piezoelectric devices that are suited for incorporating into pocket-sized cameras. Most of the electrical input energy in such lamps is lost to the gas atmosphere by the spark. Also, the electrical characteristics vary considerably from one lamp to another because of shreds of metallic combustible in the spark gap and consequent variations in effective gap length.
The use of spaced lead-in wires interconnected by a quantity of electrically conductive primer gives rise to highly predictable behavior and a well-defined electrical path through the lamp. Here again, however, relatively high-powered flash sources must be used in order to attain reliable lamp flashing.
Present state of the art flashlamps of the high voltage type make use of a bridge of initially nonconducting primer to interconnect the inner ends of the lead-in wires. Considerably higher sensitivity is attainable by this method, apparently because the breakdown and discharge follow a discrete path through the primer composition and thereby promote greater localized heating. With respect to specific construction, such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more glass sleeves, or a glass reservoir of some type. A mass of primer material contained on the bead, sleeve or reservoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metallic combustible, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at an initial fill pressure of several atmospheres. The outer surface of the lamp envelope is generally covered with a protective reinforcing coating, such as cellulose acetate, applied for example by a lacquer-dipping process.
Lamp functioning is initiated by application of a high voltage pulse (e.g., several hundred to several thousand volts, as, for example, from a piezoelectric crystal) across the lamp lead-in wires. The mass of primer within the lamp then breaks down electrically and ignites; its deflagration, in turn, ignites the shredded combustible which burns actinically.
The primers used in such flashlamps are designed to be highly sensitive toward high-voltage breakdown. Electrical energies as low as a few microjoules are sufficient to promote ignition of such primers and flashing of the lamp. This high sensitivity is needed in order to provide lamps that will function reliably from the compact and inexpensive piezoelectric sources that are practical for incorporation into modern, miniature cameras. The mechanical energy delivered to the piezoelectric crystal and thereby the electrical output energy therefrom is limited both by the size of the device and by the necessity to minimize camera vibration and motion during use.
The high degree of electrical sensitivity needed in high-voltage flashlamps gives rise to distinct problems of inadvertent flashing during their manufacture, lacquer coating, and subsequent handling. Any static charges on equipment or personnel can cause these lamps to flash. Some such lamp flashes even occur when the lamps are lying stationary in an isolated spot. Apparently, even air movements can generate sufficient electrostatic energy to promote flashing of those lamps that are by nature the most sensistive and susceptible. This problem is greatly compounded by the fact that flashlamps flash sympathetically, i.e., the radiant energy from one lamp that flashes is sufficiently intense to ignite the shredded combustible in adjacent lamps. During lamp manufacture on modern high-speed equipment, it is necessary, or at least highly expedient, at certain stages of processing, to accumulate the lamps in containers, having from about 30 to more than 2,000 lamps in a container. The problem that is encountered is that should one lamp be inadvertently ignited, all lamps in that container will sympathetically flash and be lost.
It is common practice in photoflash lamp manufacturing to dip the lacquered lamps into a bath which leaves a film of antistatic agent on their surfaces. This does much to prevent buildup of an electrostatic charge on a lamp itself by rubbing or handling. It does not, however, give a significant protection for the lamp against contact with external charges.
One means of providing the last-mentioned electrostatic protection during manufacture, processing, and handling is described in the above-referenced co-pending application Ser. No. 630,581, wherein the lead-in wires of the lamp comprise the two legs of a hairpin-shaped wire. The bight of the hairpin is disposed outside the envelope and functions to short circuit the lamp prior to use, the bight being cut to enable the lamp when inserted into an assembly.
Another means of providing electrostatic protection in a high-voltage type flashlamp is described in U.S. Pat. No. 3,873,261, wherein a conductive film is coated over the primer mass, or otherwise disposed in contact therewith, and extends to the exposed lead-in wire. In one variation, the conductive film coats a wall portion of the envelope at the base of the lamp. The filamentary combustible in the lamp is supposed to be in contact with the conductive film. I find, however, that this design has a number of shortcomings. Firstly, the conductive film would appear to be somewhat difficult to incorporate in manufacturing. Secondly, it is clear that production lamps would not consistently and reliably exhibit the desired good electrical contact between the filamentary combustible and the conductive film. Typically, one will find in practice that shredded metal foil in the lamps is shifted toward the dome of the envelope and away from the ignition structure. Thirdly, if the primer mass were coated with a conductive film, it is clear that the slightest pinhole in the primer could result in a short-circuiting condition between the inleads at a time when the lamp is supposed to be operational.
Yet another means of providing electrostatic protection in a high-voltage type flashlamp is described in U.S. Pat. No. 3,884,615 of Sobieski. In the basic lamp structure of the Sobieski patent, the two lead-in wires of the ignition mount are sealed into a doughnut-shaped glass bead which is open at both ends. The central opening in the bead is filled with a mass of primer material which bridges the lead-in wires. This construction uses the bead as a shield to keep the filamentary combustible material away from the bare lead-in wires below the bead. FIG. 4 of the Sobieski patent describes a modification in which one of the lead-in wires extends completely through the glass bead and projects above it so as to be in electrical contact with the filamentary combustible material in the lamp envelope. In this manner, the combustible material can be electrically grounded so as to reduce the possibility of accidental electrostatic flashing of the lamp. This design approach, however, also has a number of drawbacks. Firstly, the use of an ignition structure having a protruding lead-in wire would make primer application considerably more difficult. Whereas the dip method of application is generally preferred for high-volume flashlamp manufacturing, such a process would leave a continuous heavy coating of primer on the projecting lead-in wire. Consequently, a follow-up primer removal operation would be required, at added cost, in order to provide clean metal surfaces for making the desired electrical contact with the filamentary combustible material. In order to avoid the secondary wire cleaning step, the primer would have to be applied by the less desirable method of using a syringe or by daubing. A second disadvantage of the projecting lead-in wire is that the resulting higher total wire mass in the lamp imposes a significant reduction in light output. Thirdly, as both lead-in wires are disposed within a single opening in the glass bead and separated only by primer material, post-flash short circuiting can be caused by a possible welding of the lead-in wires within the bead after ignition.